This invention relates to the field of intelligent structures, i.e., structures which have sensing, processing and actuator mechanisms built in to change in response to evolving dynamic conditions or states of the structure.
An ideal intelligent structure would incorporate sets of highly distributed actuators, sensors and processors. Much work to date has centered on the development of technology for distributed actuators. However, less work has been performed on distributed sensors, and only a few pioneering efforts have dealt with distributed processors. All three of these components should be present in a system in order to implement a closed-loop shape control scheme of any complexity. The need for additional work on sensors specifically useful for intelligent structures has motivated the present invention.
The use of piezoelectric materials in intelligent structures has been studied by de Luis and by Anderson. De Luis et al. developed analytical models of piezoelectric actuators for intelligent structures, and verified these models experimentally. Analytic models were developed for static and dynamic actuation of segments of piezoelectric material either embedded in a structure or bonded to its surface. These models allow the response of the structure to actuation of the piezoelectrics to be predicted, and therefore help in efficient or optimal actuator placement. De Luis showed that although the stiffness of the structure is not strongly affected by the addition of embedded piezoelectric components, the ultimate strength of the structure is reduced. Anderson et al. also develop models for induced-strain actuation of beam-like structures. Two analytical models and one finite element based model were presented to model extension, bending and shearing deformations. In addition to this work on piezoelectric materials, electrostrictive and magnetostrictive materials as well as shape memory alloys have been investigated.
Some work has been done by Warkentin et al. on integrating electronic components in intelligent structures, who demonstrated that embedding processing and other electronic components in addition to sensors and actuators within an intelligent structure simplifies physical implementation. They described a technique for embedding silicon chips within the plies of a graphite/epoxy composite laminated structure.
Control schemes suitable for implementation on structures containing distributed actuators, sensors and processors have been developed by de Luis and by How. De Luis et al. developed optimal controllers for intelligent structures by assuming that the sensors and actuators are so numerous that they may be considered spatially continuous. This allowed use of a functional analysis approach to derive control algorithms to which an experimental implementation of a prototype of an intelligent structure and optimal controllers were derived and implemented. How et al., on the other hand, presented a new control architecture specially suited to intelligent structures with many distributed sensors and actuators. This architecture is a hierarchic one with many controllers operating at a local level, and a single global one operating to control overall motions of the flexible structure.
In the area of sensors, work at the Massachusetts Institute of Technology has been done on a variety of modal, convolving and wave sensors. Collins et al. have developed piezoelectric film sensors for the control of a two-link planar flexible robotic manipulator system. The spatial shape of the sensors is selected such that their output is proportional to a mode of the structure. These sensors make it possible to directly measure particular dynamic states of a flexible structure. Otherwise, such states would have to be determined using an estimator and some inherently inaccurate analytical model of the structure. Miller et al. investigated sensors which output a single temporal signal by convolving measurements of structural variables distributed over a finite length of a structure. The shape of the sensor determines how this convolution procedure takes place. This shape can be optimized to ensure that the sensor is specially suited for use in the control of flexible structures, because it can be designed such that the sensor output rolls off without exhibiting phase lag relative to a point sensor at its center. This would allow gain stabilization without the accompanying reduction of available gain margin.
Lee et al. have also worked on modal sensors as well as actuators. Lee and Moon developed a theory for sensing and actuation in piezoelectric laminates, in order to develop modal sensors and actuators. Such sensors, designed for one-dimensional operation have been modeled analytically and compared to experimental implementations. If such sensors were manufactured accurately, actuator or observer spillover would not exist and the modal coordinates of a structure could theoretically be measured or actuated directly. Lee, Chiang and O'Sullivan constructed modal sensors and actuators to critically damp a cantilever plate using PVF2 piezoelectric film. Using plate theory extended to include piezoelectric actuators and sensors. The sensor geometry was designed such that the sensor signal is proportional to the derivative in time of the modal coordinate of the structure. This allows a simple derivative feedback controller to be implemented. They showed that critical damping can be achieved using this controller.
A principal advantage of such shape sensors is that, relative to a point sensor at their center, they allow a rolloff in amplitude of measurement output without a change in phase. The rolloff of such sensors is not limited by Bode's Gain-Phase Theorem, because the sensors doe not make causal measurements. Bode's integral defines a relationship between the magnitude and phase of a dynamic system. The integral holds only for causal dynamics and requires that any magnitude rolloff must be accompanied by some phase lag. Point sensors are limited by Bode's theorem because they are causal and only measure present information. However, any sensor measuring strain over a finite length has access to incoming waves (future information) and outgoing waves (past information) at the same instant. Thus it has information which is noncausal with respect to its midpoint: it can sense future events. This enables it to exhibit magnitude rolloff with no associated phase lag. In the abstract, this property offers great potential for control systems based on shape sensors.
One of the shortcomings of work done to date on modal sensors is the fact that, to work well, the sensors must be able to sense deformations from all or large parts of the structure. This means that the sensor must cover a large fraction of the structure. It becomes problematic to implement several modal sensors on the same structure. Another problem is that modal sensors are typically implemented in hardware. This means that the output properties of the sensors as a function of frequency are constant. Therefore, should the shape or mass of the structure change, the mode shapes will change, and the modal sensors will not operate correctly. A third problem with the sensors currently being investigated is that it is problematic to extrapolate their design to more complex structures, such as truss structures or two-dimensional plate-like structures.
Accordingly it is desirable to develop techniques for the design of discrete distributed sensor systems that accurately estimate the quasistatic and dynamic states of an intelligent structure in a manner suitable for closed-loop control or identification.